1. Field of the Invention
The invention is concerned with a method for the manufacture of thin-walled metal tubes, in particular, the manufacture of tubes for liquefiers of freezing devices, of pipe-lines for brake fluid in automobiles, of casings for tubular electric heaters and of tubes for air heaters with low-pressure steam. In accordance with this method, a metal tape is bent, during its continuous passage through suitable forming tools, at a right angle to its longitudinal axis, and the marginal areas of the tape joined together.
2. Description of the Prior Art
The manufacture of metal tubes by having a metal tape run continuously through grooved rolls that form a split tube is known. Subsequently, the edges of the tape are welded together. If there are not any other technical reasons for a relatively considerable thickness of the wall of the tube and, therefore, for the use of much material, the thickness of the wall of the tube, and thereby the thickness of the metal tape used as the raw material, is determined by the radial pressure that will occur in use. The axial load of the tube is generally non-critical, i.e., the demand made upon the strength of the tube in the case of internal excess pressure in the lengthwise direction of the tube is only half the demand in the tangential direction.
When the split tube is manufactured, the longitudinal edges of the tubes must be pressed against one another with the aid of grooved rolls, with such force that the contact will be a permanent one, i.e., splitting open of the split tube is to be prevented. This is possible only when the tape is rolled-on-edge. Thus, the tape must be larger than the circumference of the tube to be manufactured, so as to make up for the reduction of the circumference due to the rolling-on-edges of the tube. The degree of rolling-on-edge amounts, in the case of tapes of unalloyed steel, to some 5%. Despite the rolling-on-edge, there appears, due to the action of heat during soldering, modifications of the soldered joint which affect the quality of the soldering adversely. Without any rolling-on-edge, a relatively wide split will appear, e.g., of a width of 0.2 to 0.3 mm in the case of a tape of unalloyed steel having a thickness of 0.5 mm. A split of this magnitude, however does not have the capillary effect required to draw in any solder.
In addition, the use of seamless tubes is known. With seamless tubes, a copper tube is manufactured by the extrusion of a tube of desired diameter and desired wall thickness. These tubes are relatively expensive. In accordance with the method of the present invention, a saving of more than 30% can be achieved in comparison with the cost of seamless tubes, while the properties of the tubes in accordance with the invention are equal or superior.
Furthermore, tubes manufactured in accordance with the so-called Bundy method are known. In accordance with this method, an unalloyed steel tape which is approximately twice as wide as the circumference of the tube to be manufactured, is copper-plated by galvanization.
The steel tape is then round in the form of a double-spiral, i.e., the thickness of the wall of the manufactured tube amounts to twice the thickness of the original tape. Subsequently, soldering in a soldering furnace takes place. The copper that has been applied by electro-plating is used as soldering material. In the subsequent step of the operation, in accordance with this method, the tube is then, in general, cold-tightened and, possibly, soft-annealed once more. The advantage of the Bundy-tubes as manufactured by this method, when compared to welded tubes, is to be found in their absolute tightness and, thereby, safety. It is true, however, that these tubes are relatively expensive, due to the galvanic copper-plating and the expensive manufacturing process.
In addition, there is the disadvantage that the thickness of the wall of the completed tube amounts to twice the thickness of the original steel tape, so that it is not possible to produce tubes with very thin walls.